The invention concerns an arrangement for powering a load that has non-continuous power consumption, in particular a DC motor, from a DC power supply.
In the telecommunications sector in particular, very stringent requirements exist in terms of electromagnetic compatibility (EMC). Electric motors are often used in communication technology systems, e.g. to drive fans, and the output stages of such motors are supplied with pulsed currents, e.g. for current limiting (cf. FIG. 3 below). These pulsed currents cause pulse-containing interference signals on the supply leads to such motors, and very large capacitors and inductances are needed to suppress them. Installation space is often limited, however, and the cost of such filters is high.
The filtering of high-frequency interference generally presents no major problems. Particularly stringent requirements exist, however, in the audible frequency range from 25 Hz to 20 kHz, since a humming background noise is particularly irritating when telephoning. The rotation frequency of such motors lies in the range from 25 to 200 Hz. The highest interference level is therefore reached in this frequency range. It is no longer possible to filter out this low-frequency interference using conventional filters.
It is therefore an object of the invention to make available a new arrangement for powering a load that has non-continuous power consumption, in particular a DC motor, from a DC power supply, that takes EMC requirements into account.
According to the invention, this object is achieved by means of an arrangement having a first regulator for supplying a substantially constant current, via a transistor to the DC motor and a second regulator which supplies a target value to the first regulator, based upon a voltage at the DC motor, so that the target value is automatically adapted to varying loads of the DC motor. Because a current regulator is used, the direct current that flows to an arrangement according to the present invention during operation has a very low residual ripple. During the time periods in which the load is briefly consuming little or no current, for example because of a control operation, the capacitor associated with the DC link circuit is charged by the current regulator. When the load, typically an electronically commutated motor, briefly requires more current than can be made available by the current regulator, this capacitor is partially discharged and delivers some of its energy to the load.
The DC voltage at the load thus has a small AC voltage component superimposed on it, since the voltage at the capacitor fluctuates because of these charging and discharging operations. But as long as this AC voltage component, which is critically influenced by the size of the capacitor, is small compared to the DC voltage component, it has no influence e.g. on how the motor runs.
What is obtained by means of the invention is thus an active filter that automatically compensates for low-frequency fluctuations in the power consumption of a load, and therefore corresponds very closely to the requirements for electromagnetic compatibility. It is therefore particularly suitable for use in telecommunication systems.
The arrangement is advantageously designed so that the voltage drop at the current regulator corresponds approximately to the AC voltage component of the voltage at the DC link circuit. Surprisingly, this has proven in tests to be very advantageous.
If the load is a motor and if its loading increases, the effect of the current regulator would be to decrease the voltage at the motor because its rotation speed is decreasing, and the voltage and power dissipation in the current regulator would increase accordingly. The invention is therefore advantageously refined by making the target voltage substantially proportional to the voltage drop at the transistor serving as a linear adjusting element. As a result, the current regulator automatically adapts to the power demand of a motor, i.e. if the rotation speed and voltage at the motor decrease, the target value of the current regulator is increased, and if the rotation speed and voltage rise, the target value is reduced. This is therefore an adaptive current regulator that automatically adapts to changes in power demand in a connected motor.
Further details and advantageous developments of the invention are evident from the exemplary embodiments, which are in no way to be understood as a limitation of the invention, that are described below and depicted in the drawings.